1. Field of the Invention
This invention pertains to the field of flow control devices. More particularly, it relates to motor-driven control valves and to a means of insuring continuous closure of a closed valve and means to apply added energy to the valve when it is first opened.
2. Description of the Prior Art
Many processes involve the flow of liquids and gases in tubes and pipes and require that such flows be regulated in some controlled manner. Needle valves, regulating valves and shut-off valves are examples of devices used to control the flow of these materials. Where many valves are involved or are remotely located, it has become popular to connect motors to the valves to drive them between full-open, closed and to various desired positions therebetween. Most of the motors are electric and the means of controlling them vary from simple on-off switches to feedback mechanisms coupled to computer circuitry.
An electric drive motor, usually of the reversible type, is connected to the valve stem that protrudes from the valve body. Control means are attached to the motor to drive it and the valve stem in one direction or another. Certain problems have developed in this field have not been solved so that the full utilization of motor control has not yet occurred.
For instance, one problem concerns the application of motor control to valve closure. Through repeated opening and closing of a valve, the valve seat wears, thus making the valve element that closes against the valve seat travel further into the valve housing. Since virtually all valve elements advance toward and away from the valve seat through screw threads, the wearing of the valve seat requires the valve element and stem extending therefrom to close against the valve seat at different angular positions. This means that the valve cannot be predicted to close at any particular angular position of the stem because the slightest wear on the seat will prevent the angular position from insuring that the valve is closed and allow leakage through the valve.
Accordingly, valves are set to be closed by ordering the drive motor to turn the valve stem until it stops turning, i.e., has forced the valve element fully against the valve seat. Too little motor power will not insure a fully closed valve and too much motor power may cause the valve element to mash hard against the valve seat causing increased wear or damage to the drive unit gears and other components. To avoid these situations, the prior art has established a practice of sizing the drive motor to stall at the maximum friction load needed to just close the valve. This means that the motor will just close the valve and remain in a stall condition to hold the valve shut. Such a practice not only wastes electrical power during extensive valve closure periods, but causes wear on the motor and drive gears in the form of vibration, called "chatter". Further, should a power failure occur during this valve-closure hiatus, the drive motor would cease its electrical stall and possibly allow the closed valve to drift open and allow fluid-flow through the valve when none is desired.
Also, there is the problem known as "stiction". This term comes about because of frictional buildup in the valve. While the valve stem is in motion, there is generally constant friction encountered and the load on the drive motor remains relatively uniform. That is to say, there is no buildup of forces in the valve itself and the movement from full-open to nearly full-closed position may be handled by the drive motor without difficulty. However, when the valve reaches the fully-closed position, a sudden increase in frictional load occurs in the valve stem because of tightness achieved between the valve parts as well as some friction buildup caused by flow interruption in the line. To open a fully-closed valve therefore requires the drive motor to initially overcome this rather large frictional force or "stiction". Once the valve is cracked open by the drive-motor, the stem friction drops to the relatively low value throughout the remainder of valve travel. With the prior art drive motor at stall, or maximum torque, to hold the valve closed, there is not any extra power during reverse operation to overcome this stiction and the valve often remains closed until movement is started by hand.
The prior art has attempted to utilize springs in conjunction with motorized valves to perform certain tasks, but none of these uses have dealt with the problems discussed above. In U.S. Pat. No. 4,203,573, a torsional assist spring is provided to store energy when the motor-driven valve element is rotated from its closed to its fully opened position and to release such stored energy when the valve energy is rotated from its open to its closed position to thereby assist the motor unit in driving the valve stem in the valve-closed direction. This is the opposite of what is now shown to be needed to overcome stiction created during the opening of the valve. U.S. Pat. No. 4,621,789 discloses a mechanism that is incorporated into a motorized valve in the form of a spring used to automatically open or close the valve when it becomes impossible to control the valve for energization due to power failures. This disclosure does not solve the problem of holding a valve closed while power is on.